There are known methods for adjusting the chemical composition of molten pig iron by removing at least one of such impurities as silicon, phosphorus and sulfur contained in molten pig iron in the middle of a hot-metal runner for directing molten pig iron tapped from a blast furnace into a hot-metal ladle.
The above-mentioned conventional method commonly applied so far for adjusting the chemical composition of molten pig iron by removing impurities contained in molten pig iron in the middle of the hot-metal runner, comprises charging a granular chemical composition adjusting agent for removing impurities contained in molten pig iron from a hopper arranged above the hot-metal runner into molten pig iron flowing through the hot-metal runner.
This method has however the disadvantage of a low removing efficiency of impurities because of the insufficient contact between molten pig iron and the granular chemical composition adjusting agent as a result of the fact that the charged granular chemical composition adjusting agent floats on the surface of molten pig iron and does not sufficiently penetrate into molten pig iron.
As a method for adjusting the chemical composition of molten pig iron, which solves the above-mentioned disadvantage through achievement of a sufficient contact between molten pig iron and the granular chemical composition adjusting agent and thus efficiently removes impurities contained in molten pig iron, there is known a method, disclosed in Japanese Patent Provisional Publication No. 57-200,510 dated Dec. 8, 1982, for adjusting the chemical composition of molten pig iron, which comprises:
substantially vertically arranging a lance above a hot-metal runner for directing molten pig iron tapped from a blast furnace into a hot-metal ladle so that the lower end portion of said lance is immersed into molten pig iron flowing through said hot-metal runner; and blowing, through said lance, a granular chemical composition adjusting agent for removing silicon as one of impurities contained in molten pig iron by means of a carrier gas, into molten pig iron flowing through said hot-metal runner (hereinafter referred to as the "prior art 1").
The above-mentioned prior art 1 involves the following drawbacks:
(1) The lower end portion of the lance, being immersed into molten pig iron, is susceptible to serious fusion. It is therefore necessary to frequently replace the lance, thus requiring much costs.
(2) Since the lower end portion of the lance is immersed into molten pig iron and the granular chemical composition adjusting agent is vigorously blown into molten pig iron through the lance, the blown granular chemical composition adjusting agent seriously hits the bottom of the hot-metal runner, thus the bottom of the hot-metal runner being mechanically and chemically damaged. It is therefore necessary to frequently repair the bottom of the hot-metal runner, thus requiring much costs.
As a method for adjusting the chemical composition of molten pig iron, which solves the above-mentioned drawbacks involved in the prior art 1 and permits efficient removal of impurities from molten pig iron without the risk of causing fusion of the lower end portion of the lance or damage to the bottom of the hot-metal runner, there is known a method, disclosed in Japanese Patent Provisional Publication No. 58-130,208 dated Aug. 3, 1983, for adjusting the chemical composition of molten pig iron, which comprises:
substantially vertically arranging at least one lance above a hot-metal runner for directing molten pig iron tapped from a blast furnace into a hot-metal ladle so that the lowermost end of said at least one lance is spaced apart by a prescribed distance from the surface of molten pig iron flowing through said hot-metal runner; and blowing, through said at least one lance, a granular chemical composition adjusting agent for removing impurities contained in molten pig iron by means of a carrier gas, into molten pig iron flowing through said hot-metal runner (hereinafter referred to as the "prior art 2").
According to the above-mentioned prior art 2, the lower end portion of the lance, not being immersed into molten pig iron, becomes free from fusion. In addition, because of the prescribed distance between the surface of molten pig iron and the lowermost end of the lance, damage to the bottom of the hot-metal runner caused by blowing of the granular chemical composition adjusting agent is reduced.
The above-mentioned prior art 2 has however the following drawbacks:
(1) Since the granular chemical composition adjusting agent is blown into molten pig iron without controlling the penetration depth thereof into molten pig iron, impurities cannot be removed from molten pig iron at a stable and high efficiency.
(2) Since the granular chemical composition adjusting agent is blown into molten pig iron without controlling the penetration depth thereof into molten pig iron, there is still a possibility of considerable damage to the bottom of the hot-metal runner.
The above-mentioned drawbacks are caused also when a granular chemical composition adjusting agent for further increasing the carbon content in molten pig iron is blown into molten pig iron tapped from a blast furnace to increase the carbon content in molten pig iron.
Under such circumstances, when substantially vertically arranging at least one lance above a hot-metal runner for directing molten pig iron tapped from a blast furnace into a hot-metal ladle so that the lowermost end of said at least one lance is spaced apart by a prescribed distance from the surface of molten pig iron flowing through said hot-metal runner, and blowing, through said at least one lance, a granular chemical composition adjusting agent for removing impurities contained in molten pig iron, or a granular chemical composition adjusting agent for further increasing the carbon content in molten pig iron by means of a carrier gas, into molten pig iron flowing through said hot-metal runner, to remove impurities contained in molten pig iron or to increase the carbon content in molten pig iron, thus adjusting the chemical composition of molten pig iron, there is a strong demand for the development of a method adaptable to actual operations for adjusting the chemical composition of molten pig iron tapped from a blast furnace, which permits adjustment of the chemical composition of molten pig iron at a stable and high efficiency without the risk of damage to the bottom of the hot-metal runner caused by blowing of the granular chemical composition adjusting agent. However, such a method for adjusting the chemical composition of molten pig iron has not as yet been proposed.